1. Field of the Invention
The present invention relates to a machine for carrying out the continuous contact exposure of photosensitive laminar products.
Such a machine is applicable to photographic reproduction devices in which sheets of photosensitive material are placed in direct contact with original films and then subjected to irradiation produced by light sources.
2. Description of the Related Art
Machines of this type are known which operate continuously by means of the advancement of the photosensitive materials along planar guiding slits. In particular, a planar-guide device is known and described in European patent application EP-A-117,294; said device comprises a guiding slit defined between walls constituted by planar flexible laminae which have an extremely low coefficient of friction, a planar glass plate which rests on an elastically yielding supporting planar surface, and at least one pair of traction rollers arranged upstream and possibly downstream of the guiding slit. Said rollers have, on their cylindrical surfaces, peripheral grooves which are traversed by rectilinear tabs which extend from the guiding slit, in particular from its inlet portion.
This machine facilitates the insertion of the photosensitive materials across the inlet of the slit and the advancement of said materials through the initial portion of the slit. However, the prior art machine is not free from disadvantages, which include, amongst others, the fact that the elastic reaction of the supporting planar surface providing a relatively large contact surface generates a resistance to the advancement of the materials which increases progressively and may cause the materials to travel through the slit along an irregular and non-rectilinear path and, in the worst cases, may cause the stoppage and damage thereof. This disadvantage is further increased when a plurality of original sheets superposed on the photosensitive materials are to be exposed, as frequently occurs in the montage of photosensitive plates employed in the graphic arts industry. In this case, in fact, the thickness of the original films associated with the sheets of photosensitive material becomes significant and can give rise to a total or local adhesion owing to the formation of a vacuum between the sliding surfaces. Rollers provided with grooves furthermore have, due to the grooves themselves, a small contact and thrust surface with respect to the supporting planar surface and the progressively increasing contact surface provided thereby.
In the above-mentioned exposure of a plurality of original sheets superposed on the photosensitive material, the thickness of the originals associated with sheets of photosensitive material is considerable and it is therefore necessary to make use of pairs of rollers adjustably spaced under gravity in order to provide a passage slit of adjustable variable thickness by leaving the upper rollers movable.
This gives rise to two further disadvantages, and specifically a play in the gears connecting each pair of rollers, since the axial distance of the gears can increase until the gears are no longer meshing along their pitch lines.
This drawback causes exposure defects, since the original and the photosensitive material no longer travel in register, but advance misaligned under traction.
Another disadvantage of these grooved rollers adjustably spaced under gravity arises from the fact that when the assembly formed by the original and the sensitive material are inserted across the inlet in a position thereof out of center in the widthwise direction, the rollers are no longer parallel to each other, thus causing the motion of the material to be exposed to be no longer rectilinear but directed to the right or to the left depending on the position of the rollers. Consequently, the original and photosensitive material are damaged to a degree which can even bring about the stoppage of the machine.
But the most serious drawback of these known machines consists of the need to provide rollers which are perfectly identical. In fact, in the event that the rollers of a pair are not perfectly identical, the motion of the original film is different from that imparted to the sensitive material. Consequently, an offset in images occurs. In the case of a plurality of pairs of rollers, the situation becomes even more complicated, since different localized tensions are exerted on the materials. Consequently, misalignments, defects and breakages thereof occur. It is to be noted that these tensions are consequent to differences in the peripheral speed of the rollers which are in turn caused by variations in the outer diameters of the rollers.
In fact, a difference of only one hundredth of a millimeter in the diameter of a ground rubber roller is sufficient to create a difference of 3.14 hundredths of a millimeter at every turn. After 10 turns, 3.14 tenths of a millimeter of off-register will occur, i.e. an absolutely unacceptable misalignment of the reproduced images. The above described defects are worse in grooved rollers but are also present in conventional cylindrical rollers.
Tolerances in the range described above are additionally unfeasible in the grinding of rollers covered with elastomers, due to elasticity of these materials and to the considerable coefficient of thermal expansion combined with the low melting point thereof.